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Is the LHC throwing away too much data?

By Lisa Grossman and Maggie McKee

IT’S the sort of thing that keeps particle hunters up at night. What if the Large Hadron Collider only turns up the Higgs boson and nothing else? That nightmare would leave the hunt for new physics at a dead end – a fate that could perhaps be avoided if the LHC hung onto more of its data.

Physicists celebrate even tentative signs of the Higgs (see “Still on the run“). Thought to give all other particles mass, the Higgs is the last undiscovered particle in the standard model, our most successful theory for how particles and forces interact.

The trouble is the standard model is incomplete, since it has nothing to say about gravity or dark matter. Unfortunately, no new particles have been found that might point the way to a more powerful theory (see “11 particles for 11 physics puzzles“). “It could be the situation a year from now that nothing will be found at the LHC other than the Higgs,” says Tomer Volansky of Tel Aviv University in Israel. “In that situation, we won’t really know what to do next.”

See graphic&colon; “Where the Higgs could still lie”

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At a meeting in La Thuile, Italy, last week, Volansky proposed a solution&colon; the LHC, which is at CERN, near Geneva in Switzerland, should save more of its data. The accelerator’s computers only record data when prompted by certain triggers, which are set for expected outcomes, like the particles produced when the simplest version of the Higgs decays. Volansky says we should look for signs of more exotic – and unlikely – physics, such as a new force beyond the four we already know. “We should drop our prejudice and look for anything that is possible,” he says. “If we won’t check, we won’t know.”

Others say it is not possible to save more data without extra funding or processing time. The LHC’s CMS detector, for example, takes the equivalent of 40 million pictures, each with a resolution of 1 billion pixels, every second. “Storing all the LHC data is impractical,” says Sridhara Dasu of the University of Wisconsin in Madison, who helped develop the detector’s trigger system.

Steven Lowette, another CMS team member at the University of California, Santa Barbara, agrees. Taking more data for one search means taking less for another. “The total will always need to fit in the same bandwidth,” he says.

He adds that the hunt for the Higgs will take priority this year, but that when the LHC fires up again in 2015 after a two-year upgrade, it could start to look for more exotic physics. “Searching further in overlooked corners can always come later,” Lowette says. “Bandwidth can always be reshuffled in case a new signature gains a high enough priority.”

Still, saving even a little more data would give a “taste of exotic events that may encode important information”, Volansky says. “No one has told us the answer [to what lies beyond the standard model], so sometimes we have to search in the dark.”

Now one of those groups seems to have lost the scent. At a conference last week in La Thuile, Italy, the ATLAS team reported that the statistical significance of its December signal had weakened on closer inspection.

That signal was based on only two of the Higgs’s five possible decay routes, or channels – one that decays into four particles called leptons, and the other into two gamma-ray photons. The new ATLAS result involves the remaining three channels – when the Higgs decays into two W particles, two tau particles, or a bottom quark and a bottom antiquark.

Those three channels so far show no sign of the Higgs, says Sandra Kortner of the Max Planck Institute for Physics in Munich, Germany. But Matthew Strassler of Rutgers University in Piscataway, New Jersey, points out that these three channels create some particles that ATLAS cannot detect. That means the channels carry less information about the Higgs than the two from December, which are “the ones that will be really convincing over time”, he says.

Indeed, combining all five channels still leaves room for the Higgs, says Kortner. “We still need more data to really tell.” If the LHC operates as planned, the Higgs will be pinned down by the end of 2012.